In myocardial cells from chick embryos, cultured with a method which facilitates cell differentiation and maturation, study of the ionic mechanisms responsible for heart-rate dependent changes of the transmembrane action potential in mature and immature cells will be continued by means of ion-sensitive micro-electrodes under varying concentrations of Na, K, Ca and substances which inhibit the flow of these ions (tetradotoxin, verapamil, tetraethyl ammonium). Comparison of the T wave configuration on the chest surface of dogs with the distribution of myocardial repolarization in monophasic action potentials from the ventricle will be continued by pulling fine dipole wire electrodes with one prominent and one recessed pole, combined with thermocouples, through the intact chest and various regions of the ventricles. The quality of monophasic action potentials will be improved by applying electrocautery currents to the prominent pole, and the method will be extended to T wave changes after coronary occlusion.